Electrostatographic printing operates according to the principles and embodiments of non-impact printing as described e.g., in "Principles of Non-Impact Printing" by Jerome L Johnson (1986)--Palatino Press--Irvine Calif., 92715 USA).
Electrostatographic printing includes electrographic printing in which an electrostatic charge is deposited image-wise on a dielectric recording member as well as electrophotographic printing in which an overall electrostatically charged photoconductive dielectric recording member is image-wise exposed to conductivity increasing radiation producing thereby a "direct" or "reversal" toner-developable charge pattern on said recording member. "Direct" development is a positive-positive development, and is particularly useful for reproducing pictures and text. "Reversal" development is of interest in or when from a negative original a positive reproduction has to be made or vice-versa, or when the exposure derives from an image in digital electrical signal form, wherein the electrical signals modulate a laser beam or the light output of light-emitting diodes (LEDs). It is advantageous with respect to a reduced load of the electric signal modulated light source (laser or LEDs) to record graphic information (e.g. printed text) in such a way that the light information corresponds with the graphic characters so that by "reversal" development in the exposed area of a photoconductive recording layer, toner can be deposited to produce a positive reproduction of the electronically stored original. In high speed electrostatographic printing the exposure derives practically always from electronically stored, i.e. computer stored information.
As used herein, the term "electrostatographic" also includes the direct image-wise application of electrostatic charges on an insulating support, for example by ionography.
Copying at both sides of the copying material or printing stock is favored for economic reasons.
Duplex printing is common practice in classical printing with liquid printing ink, as e.g. in offset printing of books and journals.
In electrostatography several techniques are known for forming duplex images on a final support medium such as a web or copy sheet. A survey of such techniques is given in U.S. Pat. No. 4,095,979 (Di Francesco et al. assigned to Eastman Kodak Company), which relates in particular to duplex copying by means of a photoconductive recording member.
Although most electrophotographic copiers have the capability of reproducing information on both sides of a copy sheet it is not an easy result to accomplish.
In a non-complicated embodiment described in U.S. Pat. No. 3,645,615 (Spear assigned to Xerox Corporation), the copy sheet is redirected into the feed tray of the machine after the first side of the original has been copied to receive a print of the second side of the original on the still blank side. Special paper sheet feed systems have been developed to enable duplex printing at both sides of copy sheets (see for example U.S. Pat. Nos. 4,095,979 and 4,261,661). Normally, a paper sheet reversing or turner mechanism makes the paper sheet available for a new copying cycle but now on the opposite side of the paper.
In duplex printing on web-type material likewise reversing or turner mechanisms are applied for reversing the web and feeding it into a next printing station [see for example "The Printing Industry" by Victor Strauss, published by Printing Industries of America Inc, 20 Chevy Chase Circle, NW, Washington D.C. 20015 (1967), p 512-514]. The turnaround of the web to be printed requires an additional turnaround mechanism containing one or more reversing rollers.
It has been recognized e.g. in connection with the electrostatographic duplex printer illustrated in FIG. 1 of U.S. Pat. No. 3,694,073 (Bhagat assigned to Xerox Corporation) that it will be practically impossible to maintain image quality when a toner-laden web-type receptor material comes with one or both of its toner-laden sides into contact with guiding or reversing rollers before sufficient fixing of the roller-contacting toner image has taken place.
Experiments carried out by us revealed that the problem of transfer of unfixed toner to a contacting roller (guiding or reversing roller) can be solved in the way making subject of the present invention.